System and method for producing optical circuits

ABSTRACT

A method of fabricating a plurality of composite optical assemblies is disclosed. Each optical assembly includes a first optical element and a second optical element. The method includes the steps of providing a first composite substrate that may be divided into a plurality of first optical elements and forming on an exposed surface of the first composite substrate a second composite substrate that may be divided into a plurality of second optical elements, the first and second composite substrates providing a composite structure.

[0001] The present application is a continuation-in-part application ofSer. No. 10/034,638 filed with the U.S. Patent and Trademark Office onDec. 21, 2001.

FIELD OF THE INVENTION

[0002] The invention generally relates to the fabrication of opticalcircuits, and in particular relates to the fabrication of small scaleoptical circuits such as those including micro etalons.

BACKGROUND OF THE INVENTION

[0003] Optical circuits typically include a group of individual andisolated optical elements that are arranged on a base, and that togetherperform an optical operation on a radiation beam or ray that passesthrough or is reflected by the optical elements. Each individual elementis typically supported on the base by a mounting structure that maysupport a single optical element or a group of optical elements withrespect to the base. It is customary that at least some of theindividual mounting structures will include a capability for moving oradjusting a position of the optical element in one or more degrees offreedom at a final assembly step. The adjustments may be used to alignindividual elements along a common optical axis and to direct an outputbeam or ray from the optical circuit to a desired target location. Suchalignments may also be used to direct back-reflections from opticalsurfaces away from the radiation source, e.g., a laser, so that theback-reflection do not affect the laser cavity output.

[0004] In many cases, alignment of the optical elements is so demandingthat the optical elements cannot be oriented without adjustment, oftento sub micron dimensions. Attempts to orient optical elements by simplymachining or otherwise forming an optical element mounting surface tothe tolerances required for pre-alignment of the optical element havenot been satisfactory because tolerances achievable by metal formingtechniques used to locate the optical element mounting surface fall farshort of the tolerances that are routinely achievable using alignmentand instrument feedback from, for example, a beam position detector.

[0005] For example, as shown in FIG. 1 a conventional frequency lockeroptical circuit includes a laser source 1, a lens 2, a first beamsplitter 3, a second beam splitter 4, an etalon 5, a wavelengthselective filter 6, a first detector 7, a second detector 8, and acontroller 9. Generally, a small portion of the laser signal is passedthrough each of the etalon 5 and the filter 6 to determine the precisewavelength of the signal being output from the laser. If the laser needsto be adjusted, the controller 9 automatically adjusts the laser toreturn it to the desired frequency. As shown in FIG. 1 many, if not all,of the optical elements are adjustable with respect to the base in atleast one and possibly three dimensions to ensure that the optical pathis precisely aligned. This is critical, at least in part, due to the useof the etalon.

[0006] Etalons generally include a pair of optical surfaces that areseparated from one another by a specific distance. The space or cavitybetween the optical surfaces may include any material transmissive andnon-corrupting to the light transiting the gap, which may be solid,liquid or gas as in the case of air. Sealed etalons, which provide thatthe space between the optical surfaces is enclosed, may include trappedgas or vacuum or any desired pressure. If the etalon is slightlymisaligned, its operational characteristics will completely change to adifferent frequency because the optical path length through the cavitywill change. Optical circuits, therefore, that include etalons must bevery precisely aligned to tolerances that far exceed assemblytolerances. Etalons may be used for a variety of optical applications,including variable wavelength filtering (by slightly rotating theetalon, changing its temperature, or otherwise varying the optical pathon the cavity), optical filtering of certain wavelengths, and opticalwavelength measuring systems. The precise alignment of the etalon in theoptical circuit is critical and the fabrication of optical circuitsusing micro etalons remains time consuming and expensive.

[0007] It is known that conventional optics forming methods such assurface grinding and polishing, as well as well known optical surfacemeasurement techniques such as using interferometers, can be employed toposition optical surfaces to a much higher degree of accuracy than canbe done by conventional metal forming techniques. Such optics formingmethods and measurement techniques, however, are typically unsuitablefor use in adjusting an optical element after the element is positionedon a base.

[0008] Optical circuits are utilized in many fields and are in wide usein laser systems, imaging systems, fiber optic communication systems andin optical disk devices such as compact disk memory, audio and videorecording and playback systems. An optical circuit may include as few astwo elements or may include tens of elements working together to performindividual optical operations. For example, a simple two element opticalcircuit may comprise a single surface of a flat glass plate having anoptical coating thereon. In this case the optical coating may comprisean anti-reflection coating for performing a first optical operation andthe glass having an index of refraction, which is different than air,performs a second optical operation on a beam passing through the glass.

[0009] Optical circuit examples may include beam isolators, a pluralityof beam splitters in series, beam expanders, beam directing devices e.g.utilizing a plurality of individual mirrors, wave lockers used to selecta laser output frequency etc.

[0010] There is a need therefore, for a system and method forfabricating optical circuits that are pre-aligned and pre-tested andrequire a minimum of alignment steps when installed in a larger opticalsystem. Moreover, there is need to build and align such systemsutilizing fabrication tolerances that are readily available byconventional optics fabrication techniques.

[0011] More recently, there is a need for optical circuits havingminiature optical elements, especially for use in telecommunicationsystems utilizing fiber optics. In recent laser systems, beam diametersmay range from about 0.01 mm near a work surface, to as large as 15.0 mmor more in other parts of the overall system. Accordingly, individualoptical elements such as lenses, mirrors, beam splitters, prisms,filters, and the like, may only require an optical aperture in the rangeof about 1-15 mm in diameter. Moreover, it may be beneficial that thesize of each individual element be, e.g. 1×1 mm to about 15×15 mm toreduce the size of the optical circuit or to reduce weight.

[0012] It is also a typical problem that individual miniature opticalelements are difficult to fabricate, difficult to measure, difficult tohandle, difficult to mechanically mount and align and difficult to coatwith optical coatings. In fact individual optical elements are sometimesmade larger than necessary because the larger elements can be made forless cost and are easier to manipulate. An example of a miniatureoptical element is disclosed in U.S. Pat. No. 6,276,806, which disclosesa method of fabricating micro etalons (of about 10 mm by 10 mm by 10 mm)that may be used in a variety of optical circuits. Such micro etalons,however, require very delicate and precise handling in fabricatingoptical circuits, and still must be adjusted once mounted to a base.Also, the micro etalon fabrication techniques disclosed in U.S. Pat. No.6,276,806, are not readily adaptable to the manufacture of sealed microetalons. The manufacture of sealed micro etalons remains time consumingand expensive, requiring very small parts to be assembled in acontrolled environment in which the desired gas and pressure aremaintained in a controller fashion.

[0013] There is, therefore, a further need to provide systems andmethods for fabricating miniature optical elements of high quality andlow cost and weight.

[0014] There is also a need for a system and method for economically andefficiently fabricating a large number of optical circuits such as thosecontaining micro etalons.

[0015] There is also a need for a system and method for economically andefficiently fabricating sealed micro etalons.

[0016] There is also a need for a system and method for economically andefficiently fabricating very precise micro etalons.

SUMMARY OF THE INVENTION

[0017] The invention provides a method of fabricating a plurality ofcomposite optical assemblies, wherein each optical assembly includes afirst optical element and a second optical element. The method includesthe steps of providing a first composite substrate that may be dividedinto a plurality of first optical elements and forming on an exposedsurface of the first composite substrate a second composite substratethat may be divided into a plurality of second optical elements, thefirst and second composite substrates providing a composite structure.

[0018] The invention also provides an optical circuit including aplurality of discrete optical elements that are in optical contact withone another. The optical circuit is formed, at least in part, bydividing a composite optical structure into a plurality of opticalcircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The following description may be further understood withreference to the accompanying drawings in which:

[0020]FIG. 1 shows a diagrammatic view of an optical circuit that isfabricated in accordance with the prior art;

[0021]FIG. 2 shows an illustrative view of a spacer substrate used inaccordance with an embodiment of the invention;

[0022]FIG. 3 shows an illustrative view of a first substrate used inaccordance with an embodiment of the invention;

[0023]FIG. 4 shows an illustrative view of the spacer substrate of FIG.2 placed onto the first substrate of FIG. 3;

[0024]FIG. 5 shows an illustrative view of the composite of FIG. 4 witha portion of the spacer substrate removed by grinding;

[0025]FIG. 6 shows an illustrative view of a second base substrate; usedto form an etalon in accordance with an embodiment of the invention;

[0026]FIG. 7 shows an illustrative bottom view of the second basesubstrate of FIG. 6 taken along line 7-7 thereof;

[0027]FIG. 8 shows an illustrative view of the composite of FIG. 5placed on top of the second base substrate of FIG. 6;

[0028]FIG. 9 shows an illustrative view of the composite of FIG. 8divided into a plurality of composite substrates;

[0029]FIG. 10 shows an illustrative view of a composite used to formanother composite substrate in accordance with an embodiment of theaction;

[0030]FIG. 11 shows an illustrative view of the composite substrate ofFIG. 10 divided into a plurality of composite substrates;

[0031]FIGS. 12 and 13 show illustrative views of a composite substrateof FIG. 11 being processed;

[0032]FIGS. 14 and 15 show illustrative views of sets of five compositesubstrates being processed together following the stages shown in FIGS.12 and 13;

[0033]FIG. 16 shows an illustrative view of a composite opticalstructure in accordance with an embodiment of the invention;

[0034]FIG. 17 shows an illustrative bottom view of the composite opticalstructure shown in FIG. 16 taken along line 17-17 thereof;

[0035]FIG. 18 shows an illustrative side view of the composite opticalstructure shown in FIG. 16 taken along line 18-18 thereof;

[0036]FIG. 19 shows an illustrative view of several composite opticalassemblies in accordance with an embodiment of the invention;

[0037]FIG. 20 shows an illustrative diagrammatic view of an opticalassembly in accordance with the invention in a laser frequency lockersystem;

[0038]FIG. 21 shows an illustrative graphical view of the relationshipbetween the frequency versus intensity of a signal received at theetalon detector in FIG. 20;

[0039]FIG. 22 shows an illustrative graphical view of the relationshipbetween the wavelength and the signal received at the filter detector inFIG. 20;

[0040] FIGS. 23-26 show illustrative diagrammatic views of an opticalassemblies in accordance with further embodiments of the invention;

[0041]FIG. 27 shows an illustrative view of a spacer substrate used inaccordance with a further embodiment of the invention;

[0042]FIG. 28 shows an illustrative view of a first substrate used inaccordance with a further embodiment of the invention;

[0043]FIG. 29 shows an illustrative view of the spacer substrate of FIG.27 inverted and placed onto the first substrate of FIG. 28 and thenground down to the spacer openings;

[0044]FIG. 30 shows an illustrative view of the composite of FIG. 29with a second substrate similar to that shown in FIG. 28 placed onto thecomposite of FIG. 29;

[0045]FIG. 31 shows an illustrative view of the composite of FIG. 30divided into many composite substrates;

[0046]FIG. 32 shows an illustrative view of a composite optical elementformed from the composite substrate of FIG. 31;

[0047]FIG. 33 shows an illustrative top view of the composite opticalelement shown in FIG. 32 taken along line 33-33 thereof;

[0048]FIG. 34 shows an illustrative side view of the composite opticalelement shown in FIG. 32 taken along line 34-34 thereof;

[0049]FIG. 35 shows an isometric view of a composite optical assembly inaccordance with another embodiment of the invention;

[0050]FIG. 36 shows a side view of the composite optical assembly ofFIG. 35; and

[0051]FIG. 37 shows a side view of a composite optical assembly inaccordance with another embodiment of the invention

[0052] The drawings are shown for illustrative purposes only and are notto scale.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0053] The invention provides systems and methodologies for fabricatingoptical circuits as well as for fabricating certain optical elementssuch as micro etalons. Micro etalons are in high demand in opticalcommunication applications such as fiber optic communication systems.Each etalon provides an optical element that functions as a narrowband-pass filter. The etalon may be used to transmit only a very narrowband of optical frequency by filtering out other optical frequenciesfrom an optical beam or signal having a broadband frequency orwavelength spectrum. Etalons may also be used as a background noisefilter.

[0054] The micro etalons of the present invention comprise a fixedoptical path length, or optical cavity length, and a Fabry Perotinterferometer having opposing and substantially parallel partiallyreflective surfaces bounding an air space, a solid transparent opticalmaterial or gas or vacuum filled cavity. The cavity length is defined bythe spacing between the opposing surfaces. Each surface may include anoptically reflective coating or other coating for enhancing theperformance of the etalon. The etalon cavity is constructed to transmitsubstantially all of an optical beam having a wavelength or opticalfrequency that leads to constructive interference within the cavity.Accordingly, transmission maxima occur at discrete periodic wavelengthintervals separated by a free-spectral-range (FSR) or separation betweenthe transmission peaks. The free spectral range of an etalon is governedby the following relationships:

FSR (cm⁻¹)=½ dn

FSR (nm)=λ²/2 dn

FSR (Hz)=c/2 dn

[0055] Where d is the cavity length, n is the refractive index of thecavity material, (1.0 for a vacuum), and c is the speed of light.

[0056] With reference to FIGS. 2-9, a method of forming a plurality ofmicro etalons simultaneously is described. An etalon blank is formed byinitially cutting grooves 10 into a surface 12 of a spacer substrate 14as shown in FIG. 2. The spacer substrate 14 may be formed of anultra-low expansion material such as Schott glass, and the grooves maybe ground to a depth (e.g., 5 mm) that is greater than the height of thedesired final etalon cavity length. The spacer may be a 50 mm by 50 mmsquare, having a thickness of 7-10 mm and its top surface 12 is polishedoptically flat (e.g., to within less than about 50 nm) and smooth (lessthan about 0.5 nm rms). Next, a first substrate 16 is coated withstripes 18 of a reflective material (of any reflectivity) as shown inFIG. 3. The first substrate 16 may be, for example, silicon dioxide(SiO₂), and may be 50 mm by 50 mm by 10 mm with its top surface polishedoptically flat (e.g., to within less than about 50 nm) and smooth (lessthan about 0.5 nm rms). The stripes 18 correspond to the spacing of thegrooves 10 and may be fabricated with well known mask or lithographytechniques. As shown in FIG. 4, the composite of the first substrate 16and the spacer substrate 14 is formed by turning the spacer substrate 14upside down and optically contacting it on the first substrate 16. Thesubstrates 14 and 16 are each aligned at centering comers thereon by afixed block 15 as shown in FIG. 4. The optical contacting here occursbetween the uncoated portions of the blank 16 and the top surface 12. Itis noted that the entire surface of the blank 16 may be opticallycontacted, or in certain embodiments, the contacting surfaces may beuncoated. Optical contacting occurs when two compatible materials thatare sufficiently clean, flat and smooth are brought in intimate contactwith one another and the materials are held together by molecularattraction without any intervening adhesive forming a solid and stablebond. The exposed (non-grooved) surface of the spacer substrate 14 isthen ground away to remove a portion of the substrate 14 down to theline generally indicated at 20 in FIG. 4 to expose the grooves as shownin FIG. 5. The thickness of the spacers 22 in FIG. 4 may be, forexample, 3 mm. The spacer thickness is carefully ground and polished toprovide a uniform spacer height over the entire blank since the spacerheight will become the etalon cavity length as will be described belowin more detail. Moreover, the spacer height is finally polished to thedesired cavity length as best as can be measured by shop instruments(about 0.12 μm).

[0057] A second substrate 24 as shown in FIG. 6 is then coated on itspolished surface with reflective material 26 in stripes similar to thefirst substrate 16 shown in FIG. 3 except that the second substrate 24may be thinner by a few mm than the first substrate 16 (e.g., about 7mm). In one embodiment, the opposite side of the second substrate 24 maybe coated with stripes 28 of filter material as shown in FIG. 7 thatserves as a wavelength selective filter, notch filter or other opticalfrequency selecting or absorbing coating. In other embodiments, thefilter material coating may be applied to either of the opposingsurfaces of the substrates 16 and 24 adjacent the spacer 22, or to oneof the adjacent surfaces of the spacer 22. The presence of such coatingshould not adversely affect the optical contacting of the substrates toone another.

[0058] As shown in FIG. 8, the composite of FIG. 5 is then inverted andoptically contacted onto the second substrate 24. Again, each of thesubstrates 16,22,24 is aligned at centering corners thereon by the block15. Accordingly, FIG. 8 depicts a composite optical structure 25comprising a plurality of optical circuits formed simultaneouslyaccording to the method described above. The structure 25 may beseparated into individual separate optical elements in several ways, aswill be described below. The composite structure 25 may be spatiallydivided into rows and columns of separate elements before actuallyseparating the elements such that regions of the composite structure 25may be tested before separating the elements from the compositestructure 25. For example, the optical performance of selected regionsof the composite element 25 may be individually measured in a testapparatus such as the apparatus and test method that is described inrelated U.S. patent application Ser. No. 09/872,303 filed on Jun. 1,2001, which is commonly assigned with the present application.Accordingly, the composite substrate 25 may be measured at a variety oflocations to actually measure the etalon cavity length, the FSR andother performance characteristics of what will become individual microetalon or optical circuits. One aspect of the present invention is thatif desired, the second substrate 16 may be separated from the composite25 to permit further polishing of the spacers 22 to actually adjust theoptical performance of various regions of the composite element 25. Asdescribed above, the substrate 16 is made thinner than the substrate 24so that the substrate 16 will be more flexible than the substrate 24.This allows substrate 16 to be readily removed from the composite 25without danger of breaking the optical contact between spacers 22 andthe substrate 24.

[0059] Thus according to the present invention, a sampling of locationsin representative regions of the composite element 25 may be made tocreate a map of corrective polishing that may be needed to provideuniform performance over each region of the composite element 25. Incertain embodiments, automated measurement algorithms may be used tomeasure the performance of each region of the composite element 25 andinterpolate between measured data points to accurately map the surfaceof the spacers to ensure smooth and accurate polishing of the spacers toprovide the desired uniform optical cavity length throughout. Afterfurther polishing the spacers 22, the substrate 16 is again opticallycontacted onto the spacers 22 for re-measurement of the performance ofeach region.

[0060] In various embodiments, the transmission amplitude filter coating28 maybe applied to either of the opposing surfaces on the substrate 16or 24. Coatings must be applied after the surface is polished to therequired specifications for flatness and smoothness. The optical coatingstripes are applied using a metal or other suitable coating mask. Thecoatings may be applied in a vacuum chamber by vapor deposition,sputtering or any other suitable coating method. In an embodiment inwhich the coating 28 is provided inside of the composite 25, the top andbottom substrates may be reduced in thickness once the spacer substrateis finally polished. In particular, once all regions of the compositeelement 25 are meeting the desired optical performance characteristics,the top and bottom surfaces of the composite 25 may then be ground to asmaller thickness (e.g., reduced to 1 mm to 2 mm each) as desired. Theopposing surfaces of the substrates 16 and 24 may be polished to be flatto within 50 nm and smooth to about 0.5 nm rms, and are made to beparallel to within about 50 nm.

[0061] The substrate 24 should preferably start out having a relativelylarge thickness to aid in the manufacturing process by providingrigidity (e.g., originally 10 mm), and the substrate 16 should start outbeing relatively thinner (e.g., 7 mm) so that it may be peeled off ofthe spacers to permit re-grinding of the spacers and the be easilyrepositioned on the spacers. Preferably, one or two reference edges ofeach substrate are aligned at each step of the manufacturing process.The substrates 14, 16 and 24 are joined to one another by opticalcontacting. The surfaces, therefore, must be sufficiently flat andsmooth to permit optical contacting, which occurs when the surfaces arelightly pressed together such that the air between the surfaces isforced out and a glass to glass bond is achieved at an atomic level.Although the substrates may be separated with force, there is no needfor glue or clamping or fastening during manufacturing.

[0062] As shown in FIG. 9, the composite structure shown in FIG. 8 isthen divided across the width of the structure, providing a plurality ofcomposite optical assemblies 30, each of which includes a portion of thesecond substrate 24, a pair of parallel spacers (formed by dividing eachspacer 22 in half lengthwise), and a portion of the first substrate 16.Each of these optical assemblies 30 provides a very wide etalonincluding spacers 22 and having air in the etalon space or gap that isopen at either end. The height of the air gap in the etalon isdetermined by the distance 20 (in FIG. 4) to which the spacer substratethickness is reduced. In various embodiments, if the spacers do notremain sufficiently stationary during grinding once separated from oneanother, a wax or other temporary fixing material may be introduced intothe grooves 10 in FIG. 4 prior to grinding. The wax or fixing materialmay then be removed through either the application of heat or by the useof a degreasing material. The reflective coatings 18 and 26 provide thereflective surfaces of each etalon, and each reflective coating may beapplied in 2-20 coatings to a thickness of about, for example, 1-5microns. The air space height plus the reflective coating optical pathlength determine the cavity finesse and wavelength transmissioncharacteristics of the etalon. The assemblies 30 should have a uniformthickness of each substrate along the long dimension of the assembly. Ifmicro etalon optical elements are desired without any attachedadditional circuitry, the assemblies shown in FIG. 9 (without the filtercoating 28) should be cut but in the transverse direction across each ofthe spacers to form an array of discrete micro etalons. Each of theetalon cavities in the composite shown in FIG. 4 may also be tested atmany locations along the longitudinal length of each cavity foraccuracy. For example, if the frequency of the signal is between 1525 nmand 1565 nm, then the etalon might produce, for example, four peaks atfour frequencies between 1525 nm and 1565 nm. The linear filter wouldprovide the information necessary to identify the frequency peak for thesignal. If further polishing in any area is needed, the top substratemay be removed and replaced after re-polishing as necessary. The step ofreducing the thickness of each of the top and bottom substrates to theminimum thickness (of for example 1 mm to 2 mm) should be done after thecomposite of FIG. 14 is precisely adjusted to be as optically consistentthroughout as necessary. In further embodiments, any of the surfaces mayfurther include anti-reflective coatings to reduce back reflection atthe interface of two substrates, particularly if the substrates havedifferent indices of reflection. The linear filter may be a thin filmmultiplayer coating with a spectral response linear in transmissionand/or reflection. Such a multiplayer coating is complex in the sensethat there may be 5-40 layers in total, using 2-4 materials (usuallyonly 2), which are typically silica, tantala, titania, siliconoxynitride, magnesium floride, silicon for the 1.5-1.7 μm spectralregion. The other complexity of this thin film design is that everylayer is of a different thickness. These coatings are difficult tomanufacture and require tight control of the process parameters of thecoating chamber.

[0063] As shown in FIGS. 10-15, another optical element may be formed asa pair of beam splitters. As shown in FIG. 10, a composite 32 of threelayers of SiO₂ material, measuring 50 mm by 50 mm by 8 mm is dividedinto elongated portions 34 as shown in FIG. 11. Each portion 34 is thenplaced into a wedge block 36 and the portion 34 is cut along the linegenerally indicated at 38. The piece is then placed onto a flat block 40and the opposite corner of the piece is then cut along the linegenerally indicated at 42. The remaining portions of the pieces 34 arethen collected, and are arranged together for stability and placed on ablock 44 as shown in FIG. 14 where they are further cut along a linegenerally indicated at 46. The pieces are then inverted and placed onanother block 48 where they are cut along the line generally indicatedat 50 as shown in FIG. 15. In various embodiments, the inside surfacesof any of the substrates that form the composite 32 may be uniformlycoated to provide specific transmission characteristics. Without anysuch coatings, the beam splitters may provide about 4% to 5% reflectionin the direction transverse to the direction of the incident signal.

[0064] Each fully trimmed portion 34 may then be optically contactedwith a composite substrate of FIG. 9 as shown in FIG. 16 to provide acomposite optical structure. The matching of the lengths (and possiblywidths) permits the two substrates to be easily aligned when combined.The sizes of the original substrates 14, 16, 24 and 32 are chosen to beconsistent to permit the composite optical structures to be easily andaccurately aligned with one another. Certain additional optical elementsmay be added to the structure, such as lenses 52 and signal detectors 54and 56 (such as InGaAs detectors) as shown in the side and bottom viewsof FIGS. 17 and 18. As shown in FIG. 19, each composite opticalstructure may then be divided into a plurality of optical assemblies 58,each of which provides a complete optical circuit. The opticalassemblies may each be, for example, 6 mm by 4 mm by 3 mm.

[0065] Each optical assembly 58 may be coupled to a laser 60, a thermoelectric cooler 62, and a controller 64 as shown diagrammatically inFIG. 20. The system of FIG. 20 provides a laser frequency locker circuitthat detects any drift in output frequency of a laser and corrects thelaser accordingly. In particular, a laser 60 outputs a signal 66 that ispassed through a lens 52 and then divided at a first beam splitter 68where a portion (e.g., 5%) of the signal is directed toward the etalon,which includes a pair of parallel flat surfaces coated with thereflective material 18 and 26. The laser signal 60 that passes throughthe beam splitter 68 encounters another beam splitter 70, and a portion(e.g., 5%) of the signal is diverted toward a wavelength selectivefilter formed by the path through the etalon spacer and including thefilter coating 28. In various embodiments, the filter coating may beapplied on any of a variety of the surfaces with in the etalon, such ason either side of the spacer 22. The diverted signal from the etalon isreceived by the etalon detector 54, and the diverted signal from thefilter 56 are both coupled to the controller 64. The output of thecontroller 64 is coupled to the thermoelectric cooler 62 for the laser,and cause the laser temperature to be adjusted to correct for anyvariation in frequency. The output signal of the system is provided at72. The etalon filter provides a transmission versus frequency responseas generally shown in FIG. 21 where a number of frequency peaks areprovided, and the filter provides a transmission versus frequencyresponse as generally shown in FIG. 22 that is linear. The combinationpermits the system to discern which side of which peak the etalonresponse lies on.

[0066] The system shown in FIG. 23 is similar to the system as shown inFIG. 22 except that the top surface of the etalon is cut at an angle,and the bottom surface of the beam splitter element is also cut at anangle that complements the angle of the etalon. These cuts are performedby placing the assemblies 30 and the trimmed portions 34 on an angledbock and then cutting the assemblies prior to joining the compositesubstrates together as shown in FIG. 16. The non-normal angle at theinterface of the optical elements prevents back reflection of the signalat the interface of the beam splitter element and the etalon fromtraveling along the optical path back to the laser. In furtherembodiments, other surfaces such as the exposed surface of either of thesubstrates 34 and/or 24 may also be cut to the wedge shape to furtherinhibit back reflection and to permit the thickness of the respectivesubstrate to be uniform.

[0067] As shown in FIG. 24, a system 80 in accordance with anotherembodiment of the invention includes a laser 60, cooler 62 andcontroller 64 as discussed above. The system 80, however, inputs thelaser signal through a different side of the assembly to an initial beamsplitter 82. This permits the signal from the second beam splitter 70 toalso be captured and detected at a detector 84, the information of whichis transmitted to the controller 64. The system output signal isprovided at 86. The second beam splitter 70 functions as a filter beamsplitter providing a signal intensity response that is the inverse ofthe response received at the detector 56. In particular, the responsewill be in the inverse of the response shown in FIG. 22, such that thesum adds to unity.

[0068]FIG. 25 shows another embodiment of a system 90 in accordance withanother embodiment of the invention in which a laser signal from laser60 is passed through a beam splitter 92 and a portion is diverted to anetalon filter and then through a filter 94 to a detector 96 as shown. Aportion of the signal is also reflected back toward the beam splitter92, and a portion of the signal is then received at another detector 98.The system output signal is provided at 99. If the filter 94 is an airgas etalon having a spacing of 4.8 microns, the filter response may beslightly sinusoidal. Such a system may have a free spectral range of atleast 10 THz and a finesse of about 3.5. It operates in a mode similarto the linear filter, and provides a mathematically well-defined Airyfunction response of the etalon to locate any InternationalTelecommunications Union (ITU) channel within a given range. In thenotch or absorption filter mode of operation, the laser is tuned over awavelength region containing an absorption response. This is either inthe form of a temperature-stabilized thin film notch filter or anabsorption in the etalon plate (e.g., Erbium-doped) or a gas-filledetalon cavity (e.g., acetylene). As the laser is tuned over thiswavelength reference, the absorption dips are registered and the elatonfringes are counted consecutively from that point onwards. The number ofthe ITU channel may be ascertained provided the etalon is tuned andstabilized to the ITU grid. In this design, the laser beam is tapped offby a BS 92 and passes through the etalon in series with a linear oretalon filter 94. The transmission signal at detector 96, and thereflection signal at detector 98 are the product of the etalon andfilter 94 response functions, i.e., the intensity of the etalon peaks ismodulated by the transfer function of 94. This aspect differs from thedesign shown in FIG. 24 where the etalon and linear filter responses aremonitored separately. As in FIG. 24, the sum of the signals at 96 and 98is a measure of the total signal power, and all signals are normalizedto this value. The difference of the signals at 96 and 98 allows thedetermination of the wavelength, i.e., peak, through a specifiedcalibration procedure. The circuit of FIG. 25 provides a relativelysimple circuit that uses only two detectors in an efficient fashion.

[0069]FIG. 26 shows another embodiment of a system 100 in accordancewith another embodiment of the invention in which a laser signal ispassed through a beam splitter 102 where it is diverted toward an etalon104. Some of the etalon signal is reflected back toward the beamsplitter 102 and passed to detector 106 as shown. The laser signal thenpasses through another beam splitter 107, and a portion of the signalpasses through a linear filter 108 and is received by a detector 110.The system output is provided at 112. This device is based on the etalonpeak counting method. As described above, the detector responses at 105and 106 provide the total signal power. The beam is tapped again bysplitter 107 and passes through a filter 108. The filter 108 may be athin film narrow band pass filter (e.g., a notch or peak filter), adoped glass plate exhibiting absorption (e.g., by erbium), or a gas(e.g., acetylene) filled cavity exhibiting strong and sharp absorptionlines. As the laser is tuned over a given frequency band encompassingthe spectral features of filter 108, detector 110 will detect spectraldips/peaks whose frequency is accurately known, intrinsically as in thecase of acetylene or erbium, or temperature controlled in the case of athin film filter, thereby providing an accurate frequency reference. Asthe laser is scanned further, detector 105 picks up the etalon peaksthat are counted sequentially.

[0070] FIGS. 27-33 show various stages of fabricating a sealed etalon. Aspacer substrate 120 is provided with a plurality of openings or holes122 as shown in FIG. 27, and this substrate is inverted and placed ontop of a base substrate 124 (shown in FIG. 28) onto which reflectivecoating portions 126 have been deposited in a pattern of small circlesmatching the openings 122. Again, a block 15 is used for alignmentpurposes. The portions 126 match the openings 122 when the substratesare combined, and the exposed surface of the spacer substrate may beground to a reduced thickness as discussed above in connection with FIG.4 as shown in FIG. 29. A top substrate 128 similar to the substrate 124with the reflective coatings 126 is then placed on top of the spacersubstrate 120 as shown in FIG. 30, and the composite may be divided intoelongated portions 130 as shown in FIG. 31. As shown in FIG. 32 eachportion 130 may be divided into individual sealed micro etalons, each ofwhich is generally cylindrically shaped as shown in the top view of FIG.33 and the side view of FIG. 34.

[0071]FIGS. 35 and 36 show another embodiment of an optical assembly 130in accordance with the invention. The optical assembly 130 may be usedas a laser end facet wavelocker and includes a collimating lens 132,abeam splitter 134 and an etalon cavity formed between etalon surfaces136 and 138. Each of the length, width and height dimensions of theoptical assembly may be less than about 1 cm, and preferably may be lessthan ½ cm, with the etalon cavity length determined by the etalon freespectral range (FSR) as discussed above.

[0072] The optical assembly 130 may be formed in accordance with themethod discussed above with reference to FIGS. 1-19 except that the beamsplitter optical element includes one instead of two beam splitters ineach of the elongated portions 34. The assembly 130 is formed bycombining optical elements into a composite optical structure that isthen diced up to create a plurality of optical assemblies. As shown inFIG. 36, the optical assembly 130 also includes a pair of detectors 140and 142 for capturing signals that emanate from each of the twodirections from the beam splitter 134 as shown.

[0073] During use a back reflected signal from, for example asemiconductor laser, is collimated by the lens 132 and split by the beamsplitter 134 into a first beam (e.g., 70%) that travels through theetalon formed by surfaces 136 and 138 to the first detector 140, and asecond beam (e.g., 30%) that travels through the beam splitter 134 tothe second detector 142. The circuit of the assembly 130 does notinclude a wavelength selective filter.

[0074]FIG. 37 shows a side view of an optical assembly 150 including acollimating lends 152, a beam splitter 154, etalon surfaces 156 and 158,and detectors 160 and 162. The assembly 150 is similar to the assembly130 shown in FIG. 36 except that the extended surfaces that form theetalon surfaces 156 and 158 are slightly non-normal to the direction ofthe signal beam from the beam splitter 154. This prevents backreflection into the laser from the etalon surfaces 156 and 158. Thesurfaces 156 and 158 are parallel with respect to one another and may beformed by using wedge blocks as discussed above with reference to FIG.23. The optical elements and lens and detectors may be combined usingoptical contacting as discussed above and/or a wide variety of adhesiveor other conventional bonding methods.

[0075] The substrates that form the end plates and spacers of the etalonmay be formed of a material having a low thermal expansion and highrefractive index homogeneity such as glass or glass ceramic. Forexample, the glass ceramic ZERODUR sold by Schott Glas of Mainz Germanymay be used.

[0076] The invention provides stable alignment of micro optics in thatoptical contacting may be employed requiring no intermediate adhesive toaffect dimensional stability. The devices of the invention are accuratebecause alignment of the long dimensions of the sub assemblies permitsthe optical elements to be easily and accurately aligned. The inventionalso permits bulk handling of the devices, i.e., simultaneous assemblyof multiple devices. In particular, devices may be simultaneouslyassembled having optical paths in more than one plane. The bulk handlingalso facilitates testing in that sampling may be done while componentparts are part of a monolithic structure and properties of inter-siteparts may be accurately inferred. Further sealed or enclosed opticalpaths may be achieved which inhibit contamination. In fact, a completeenclosed optical circuit may be provided for signal processing.

[0077] Although the above disclosed examples relate to wavelengthlocking circuits, optical circuits in accordance with the invention maybe used for a wide variety of purposes, including but not limited totransmitters, multiplexors, interleavers, isolators, dispersioncompensators, circulators, optical switches, tunable filters, wavelengthrouters, demultiplexors, attenuators, receivers, micro-optical wavemeters, absolute wave meters, interferometers, micro-interferometers,and any other frequency or wavelength measuring component.

[0078] Also, micro circuits made in accordance with the invention thatinclude etalons, beam splitters, filters, and/or sensors etc. need notonly be used in the traditional optical infra-red through ultra violetwavelengths. Those skilled in the art will recognize that using suitablematerials these constructions can be used to create circuitsutilizing,any wave like radiation such as for example, microwave orX-ray wavelengths.

[0079] Those skilled in the art will appreciate that numerousmodifications and variations may be made to the above disclosedembodiments without departing from the spirit and scope of the presentinvention.

What is claimed is:
 1. An optical circuit comprising a at least two ofoptical elements that are in contact with one another, said opticalcircuit having been formed, at least in part, by dividing a compositeoptical structure into a plurality of optical circuits.
 2. The opticalcircuit as claimed in claim 1, wherein said optical circuit provides anend facet wavelocker.
 3. An optical circuit comprising a plurality ofdiscrete optical elements that are optically contacted with one another,said optical circuit being defined by a length, a width and a depth thatare each less than about 1 cm.
 4. An optical circuit comprising aplurality of discrete optical elements that are optically contacted withone another, said optical circuit being defined by a length, a width anda depth that are each less than about ½ cm.
 5. An optical circuitcomprising a plurality of discrete optical elements that are opticallycontacted with one another, said optical circuit including at least onebeam splitter and an etalon.
 6. An end facet wavelocker circuitcomprising a plurality of discrete optical elements that are opticallycontacted with one another, said optical circuit including at least onebeam splitter and an etalon and being defined by a length, a width and adepth that are each less than about 1 cm.
 7. A method of fabricating aplurality of composite optical assemblies, each of which includes afirst optical element and a second optical element, said methodcomprising the steps of: providing a first composite substrate that maybe divided into a plurality of first optical elements; and forming on anexposed surface of said first composite substrate a second compositesubstrate that may be divided into a plurality of second opticalelements, said first and second composite substrates providing acomposite structure.
 8. The method as claimed in claim 1, wherein saidmethod further includes the step of dividing said composite structureinto a plurality of composite optical assemblies, each of which includesa first optical element and a second optical element.
 9. The method asclaimed in claim 8, wherein said composite optical assemblies eachincludes an end facet wavelocker.
 10. The method as claimed in claim 8,wherein said method further includes the step of removing a generallywedge-shaped portion of material from said exposed surface of said firstcomposite substrate prior to forming said second composite substrate onsaid first composite substrate.
 11. The method as claimed in claim 8,wherein said method further includes the step of removing a generallywedge-shaped portion of material from said exposed surface of said firstcomposite substrate prior to forming said second composite substrate onsaid first composite substrate.
 12. The method as claimed in claim 8,wherein said composite optical assemblies are each defined by a length,a width and a depth of less than about 1 cm.